Allorecognition, or the response of an individual to non-self antigens expressed on conspecific tissues, forms the basis for the rejection of life-saving organ transplants in humans. Despite extensive studies in mammalian models of allorecognition, safe and effective methods for preventing graft rejection are still lacking. In this grant application, we propose to extend our fundamental investigations into the mechanisms of allorecognition in an invertebrate model organism, Hydractinia symbiolongicarpus. Hydractinia is a colonial marine invertebrate that displays an unambiguous fusion/rejection response regulated by a highly specific allorecognition system. Through funding provided by an exploratory (R21) grant from the NIH, we have already identified two candidate genes, alr1 and alr2, that dictate fusion/rejection phenotypes in Hydractinia. We have also established reproducible experimental systems to track chimerism in histocompatible and incompatible fusion partners and investigate its effects on allotolerance. The specific aims of the current proposal are to (1) functionally characterize alr genes, gene products, and interacting proteins;(2) further the genetic characterization of Hydractinia allorecognition;and (3) assess the effect of genetic distance/degree of histo-incompatibility on the stability of chimerism and allotolerance. The contribution of [unreadable]simple[unreadable] model organisms to detailed understanding of biological processes in higher metazoans is underscored by discoveries made in Drosophila (host defense), C. elegans (aging), and zebra fish (organogenesis). The molecular characterization of the Hydractinia allorecognition system would establish this invertebrate organism as an invaluable model for probing fundamental processes relevant to solid organ, bone marrow, and stem cell transplantation.